1. Field of the Invention
The present invention relates to a read only memory. More particularly, the present invention relates to a semiconductor memory and an NROM memory cell therein having two independently controllable gates.
2. Description of Related Art
A conventional nitride read only (NROM) memory cell comprises a P-type substrate on which is formed an oxide/nitride/oxide (ONO) stacked layer structure, with the silicon nitride layer serving as an electron trapping layer. A control gate structure of a conducting polycrystalline layer is formed on the silicon oxide/silicon nitride/silicon oxide layer. An N+ source region and an N+ drain region are located in the substrate on either side to the gate structure.
The conventional NROM cell can store two bits of information, one bit of information being trapped as the presence or absence of negative charges in the trapping layer at the side of the source region and one bit of information being trapped as the presence or absence of negative charges in the trapping layer at the side of the drain region. The bit information at the source and the drain regions is separately read by detecting the presence of absence of current flowing between the source and the drain when appropriate voltages are applied to the gate, the source and the drain. However, in reading one of the two bits of data in the conventional NROM cell, the magnitude of the current that travels between the source and the drain regions may be affected by the presence or absence of the other bit of data. This is called the second-bit effect. The presence of the second-bit effect makes less reliable, the reading of the contents of the cell.
In addition to the second bit effect, when NROM cells are configured in an array, leakage currents from unselected cells may effect the reading of the contents of a selected cell. Such an effect is commonly called an array effect. Accordingly, it would be desirable for an NROM cell to have the capability of storing two bits of data where the presence or absence of one bit of the data does not influence the reading of the other bit of data, and where leakage currents generated from unselected cells in an array, which could effect the reliability of reading the contents of the selected cell, are not generated.